1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a network printer and a digital complex machine.
2. Description of the Related Art
In recent years, there has been put to practical use an image forming apparatus such as a digital color copying machine which offers a precise copy of an original image and has a high picture quality. Along with this, a banknote, a valuable certificate or the like may be copied and easily counterfeited. As a forgery prevention technique for coping with such counterfeiting, a method is devised of: deciding whether or not an original that is set on an original-paper table is an original whose copy is prohibited, using an image processing such as pattern matching; and stopping copying it forcedly. However, in order to make a pattern match, all the characteristics of a copy-prohibition original such as a banknote, a valuable certificate, a traveler's check, a bank-account check and an official document all over the world have to be stored in a storage unit. This requires a large-capacity storage unit. Besides, if pattern matching is conducted every time each original is read, the time taken for reading them becomes far longer. This makes it practically difficult to adopt the above described forgery prevention technique in an image forming apparatus.
Therefore, a conventional image forming apparatus is known which has a function of: writing, in a copy, a specific pattern which expresses, using a dot pattern hard to visually confirm, information for identifying an apparatus which printed the corresponding copy, for example, information (i.e., the apparatus identification information) including the apparatus' manufacturer name, type name, serial number, copying date or the like; and thereby, identifying the apparatus which printed the corresponding copy.
However, in this image forming apparatus, there is a disadvantage in that such a specific pattern becomes conspicuous in a highlight part. Therefore, Patent Document 1 (e.g., in Paragraph [0008] of Japanese Patent Laid-Open No. 6-62217 specification) discloses an image forming apparatus which is capable of lowering the intensity of a specific pattern in a highlight part where a person is sensitive to a shade difference and heightening the intensity of the specific pattern in a shadow part where a person is insensitive so that the person can read the specific pattern certainly within any shade range while making the specific pattern inconspicuous.
In addition, in the above described conventional image forming apparatus, such a specific pattern is usually printed using a specific toner with a light color, for example, a yellow toner, which is hard to visually confirm. But this presents a disadvantage in that the consumption of this yellow toner increases. Therefore, Patent Document 2 (Japanese Patent Laid-Open No. 2005-254774 specification) discloses, taking into account the fact that a sheet of recording paper such as reused paper whose reverse side is not blank is extremely unlikely to be used for counterfeiting, an art of extracting the fact that the recording paper's reverse side is blank or not, and if it is not blank, not writing the specific pattern, so that this specific toner will be less consumed.
Furthermore, different from a sheet of paper having a regular size such as A4 and B5, a sheet of copy-prohibition original paper such as a banknote can be set in various positions on a sheet of contact glass. Hence, if the writing area of such a specific pattern is set to a specific area on recording paper, then the specific pattern cannot be written within the area where the copy-prohibition original is printed.
However, in the arts of Patent Document 1 and Patent Document 2, no measure is taken to print the specific pattern within the area where such a copy-prohibition original is printed. Therefore, in order to print the specific pattern within the area where the copy-prohibition original is in print, the specific pattern needs to be repeatedly printed over the whole surface of the recording paper. This raises a disadvantage in that the consumption of a specific toner increases.